1. Field of the Invention
The invention relates to electronic image capture devices, and, in particular, to an optical arrangement that enables the device to simultaneously image up to four fingers on a rectangular platen surface having an aspect ratio greater than the aspect ratio of an imaging camera sensor array.
2. Description of the Prior Art
Until recently, law enforcement agencies and other security conscious entities made fingerprint records of individual subjects by inking the subject's fingers and then pressing or rolling the fingers onto a card with designated spaces for the various prints. A typical, so called "ten-print" card 10 of fingerprints, illustrated in FIG. 1, includes a "rolled" impression 12 of each fingertip, "slap", or plain, impressions 14 of the four fingers on each hand, and plain impressions 16 of each thumb.
A rolled impression is taken with an ink and paper system by placing a side of the inked fingertip on a selected space delineated on a standard sized fingerprint card, and rolling the fingertip from fingernail to fingernail once across the space. This process is repeated for each finger and thumb. A four-finger plain impression is taken by simultaneously pressing the inked four fingertips of one hand on a selected space on the fingerprint card. Slap imprints of the thumbs are taken separately. The roll impression spaces 18 are 1.6 inches wide by about 1.5 inches high. The four-finger plain impression spaces 20 are about 3.2 inches wide by about 2 inches high. The spaces 22 for the plain thumb impressions are about 0.8 inches wide by about 2 inches high.
Alternatively, agencies can use electronic fingerprint imaging systems to capture fingerprint images. The electronic systems have several advantages. An electronically generated image can be reproduced, stored or retrieved easily. Persons at one location can quickly transmit an accurate electronic image to another location by facsimile or by transferring a file between computers that are linked together by, for example, a modem connection. Obtaining an electronic image is quick and clean. No ink needs to be applied or removed from a subject's fingers, as with prior art paper and ink systems.
Prior art electronic fingerprint imaging systems typically include a single optical system for both the rolled and the plain images, or a pair of independent optical systems, one for rolled images and one for a plain four-finger image, contained in a cabinet that is configured to exclude stray light and dust. Each optical system may have one or more imaging cameras dedicated to that system. Each camera typically includes a CCD array configured with a 4:3 aspect ratio, i.e. with a light sensing surface having a width that is one third larger than its height. A typical CCD array of this type will have about 485 horizontal lines with 400 to 800 pixel elements in each line.
Each optical system includes a transparent platen prism mounted in the cabinet. Each platen prism has a finger receiving surface that is exposed, typically at the top of the cabinet. A light source in the cabinet directs light through a face of the optical platen prism to the finger receiving surface at an angle .phi. that is greater than the critical angle for total internal reflection at the platen/air interface. Light that illuminates covered portions of the finger receiving surface of the platen prism, such as are in contact with skin features, for example fingerprint ridges, is not internally reflected. Light that illuminates bare portions of the finger receiving surface of the platen prism, such as are located below valleys between fingerprint ridges, is internally reflected.
A reflection image of the fingerprint is thereby formed. Dark portions of the reflection image are representative of raised skin features; light portions of the reflection image are representative of indented skin features and uncovered regions of the finger receiving surface that surround the finger. The reflection image is transmitted through a second side of the optical platen prism and then directed to the imaging camera or cameras.
Some systems use an alternate technique. A finger receiving surface of the platen prism is illuminated from below or in a direct fashion wherein the light penetrates the finger receiving surface. The finger receiving surface is still imaged at an angle above the critical angle for the platen/air interface. Where the finger contacts the platen, the light will diffuse into the platen prism and a portion of this diffused light will be imaged. Light directed from above the platen/air interface, where there is no contact, is not diffused back into the platen prism and cannot be imaged. A light image of the fingerprint appears in a dark background. This is referred to as a "weak image" since only a small fraction of the illumination is imaged. The other form, the reflection image, is referred to as the "strong image."
Since a four-finger plain impression is about 3.2.times.2 inches, it requires a final image signal with an aspect ratio of about 1.6:1. Prior art systems typically illuminate the finger receiving surface through a long side at the front of the platen prism. Since the angle .phi. at which the contact surface is viewed is typically about 45.degree., the aspect ratio will be increased by about 1/cos .phi. to about 2.2:1. Most CCD camera arrays in commercial use have an aspect ratio of 1.33:1, so they are not very effective for imaging a fingerprint reflection image having an aspect ratio of 2.2:1.
The prior art solution has been to obtain partially overlapping images off side-by-side regions of the finger receiving surface. These systems then electronically stitch together the signals representing the overlapping images to provide a signal representing an image of the entire finger receiving surface. A first type of system uses a single camera and moves the optics on a carriage to sequentially take a series of images covering the entire finger receiving surface. A second method uses up to four cameras in a fixed optical system to simultaneously take images of adjacent regions of the finger receiving surface. Each approach requires complex data processing to perform the image stitching.
Therefore, it is an object of the invention to provide a fingerprint image capture device that can obtain a four-finger plain image with fixed optical components and a single imaging camera.
Another object of the invention to provide an optical system for a fingerprint image capture device that matches the aspect ratio of the imaging camera to the aspect ratio of the finger receiving surface of the optical platen prism for four-finger plain images.
Yet another object of the invention to provide a fingerprint image capture device that can capture the entire field of a four-finger plain image with a single camera exposure.